1. Field of the Invention
The invention relates to a short arc ultra-high pressure mercury lamp. The invention relates especially to a discharge lamp used as a light source for a liquid crystal display device in which the light source is an ultra-high pressure mercury lamp filled with at least 0.15 mg/mm3 of mercury, and in which the mercury vapor pressure during operation is greater than or equal to 110 atm. The discharge lamp can also be used in a projector device such as a digital light processor (DLP) or the like having a digital micro mirror device (DMD).
2. Description of the Related Art
In a projector device of the projection type, there is a demand for illumination onto an image device in a uniform manner and with adequate color rendering. The light source is therefore often a metal halide lamp which is filled with mercury and a metal halide. Furthermore, recently smaller and smaller metal halide lamps and point light sources are being produced for such use and these lamps have extremely small distances between the electrodes.
Instead of metal halide lamps, discharge lamps with an extremely high mercury vapor pressure, for example with 150 atm, have been recently proposed. In these lamps, the broadening of the arc is suppressed (the arc is compressed) by the increase of the mercury vapor pressure and a substantial increase of light intensity is realized. Lamps of these ultra-high pressure discharge type are disclosed, for example, in Japanese Patent document HEI 2-148561 (see the English equivalent—U.S. Pat. No. 5,109,181) and Japanese Patent document HEI 6-52830 (see the English equivalent—U.S. Pat. No. 5,497,049).
When an ultra-high pressure mercury lamp is used, a pair of opposed electrodes are positioned with a spacing distance of at most 2 mm in a silica glass arc tube filled with at least 0.15 mg/mm3 of mercury and halogen in the range of 1×10−6 μmole/mm3 to 1×10−2 μmole/mm3. The main purpose of adding the halogen is to prevent devitrification of the arc tube. However, when constructed in this manner a so-called “halogen cycle” arises.
In the above described ultra-high pressure mercury lamp (hereinafter also called only a “discharge lamp”), the phenomenon occurs that, in the course of operation, projections are produced on the electrode tips. This phenomenon is not entirely clear, but the following can be reliably determined.
The tungsten which is vaporized from the high temperature area in the vicinity of the electrode tip during lamp operation combines with the halogen and residual oxygen which are present in the arc tube. When bromine (Br) is added as the halogen, it is present in the form of a tungsten compound such as WBr, WBr2, WO, WO2, WO2Br, WO2Br2 or the like. These compounds decompose in the gaseous phase in the high temperature area in the vicinity of the electrode tip and yield tungsten atoms or cations. Due to thermal diffusion (i.e., diffusion of the tungsten atoms which are moving from the high temperature area in the gaseous phase (=arc center) in the direction of the low temperature area (=vicinity of the electrode tip)) and due to the fact that in the arc the tungsten atoms are ionized, i.e., as cations, the tungsten cations are pulled during operation of the electrode as a cathode by the electrical field in the direction to the cathode. The tungsten vapor density in the gaseous phase in the vicinity of the electrode tip therefore becomes high, which results in precipitation on the electrode tip to form the tungsten projections. The formation of the above described projections is disclosed, for example, in Japanese Patent document 2001-312997 (see the English equivalent—U.S. Pat. No. 6,545,430).
FIGS. 7(a) and 7(b) each schematically show the electrode tips and projections. In the FIGS. 7(a) and 7(b), the electrodes 1, as a pair, are formed of a spherical part 1a and a shaft 1b. On the tip of the spherical part 1a, a projection 2 is formed. In the situation in which, at the start of lamp operation, there is no projection, the projections 2 are produced during the subsequent operation, as are shown in the Figures. These projections 2 cause an arc discharge A.
However, the formation and growth of the above described projections have some disadvantages.
Fluctuation of the Lamp Voltage—The above described projections are not present in the lamp when it is manufactured, but the projections are produced and grow in the course of subsequent operation. The formation of projections also depends on the types of lamps and the like, but after for example 80 to 100 minutes have passed, the growth is essentially ended. During formation of these projections and after usage is ceased for the first time, the distance between the electrodes in the course of operation has been shortened. Additionally, the operating voltage of the discharge lamp is reduced.
Reduction of the Light Utilization Efficiency—The above described projections do not always form on the electrode axis. If, for example, as in FIG. 7(a) they are formed along the electrode axis L, there is little or no disadvantage. However, there are also situations in which the projections are formed which diverge from the electrode axis, as in FIG. 7(b). In this situation, the arc position also deviates from the electrode axis L. The major disadvantage then occurs in that for an optical system designed as a point light source, the degree of light utilization decreases.